The present invention relates to coating a moving substrate or web with a coating fluid or liquid by passing the web into contact with a liquid pool, so that a portion of the pool is carried away as a thin coating on at least one side of the web.
The present invention finds utility, for example, in the manufacture of flexible xerographic photoconductor media and flexible magnetic recording media. In such a process, a web of biaxially oriented polyethylene terephthalate, acetates, polyolefins, or other conventional polymeric films, which are in the range of 0.0015 inch to 0.005 inch thick, and are from 12 inches to 48 inches in width, are coated with a layer of coating liquid, or ink, which is in the range of from 0.00025 inch to 0.0055 inch thick.
A typical photoconductor ink is a Newtonian liquid, whereas a typical magnetic recording ink is a non-Newtonian liquid. The present invention has utility in the coating of both general classes of liquids.
A great number of means and apparatus exist which operate to coat such liquids onto a moving web. For example, an excess amount of liquid may be applied, with excess liquid thereafter being removed by operation of a doctor knife. Also, a roller may be used to transport liquid from a pool to the passing substrate; or the liquid may be extruded in a thin layer directly onto the passing web. The substrate may be routed into a reservoir of coating liquid, either as a free-running web, or while being guided by a backup roller, and air brushes or resilient wipers can be used to thereafter remove excess coating liquid from the substrate.
Another class of coating apparatus provides a pool of liquid at a coating nip through which the substrate passes. As a result, the substrate exits the coating nip carrying a thin layer of the coating liquid.
In all cases, the coating liquid is a consumable, and it must be replenished or resupplied to the pool which is immediately at the coating nip or coating interface. Generally, it is an advantage to maintain only a small amount of coating liquid at this pool, and yet the pool must be large enough to supply a uniform quantity of liquid across the length of the coating interface. These coating inks are usually dispersions which contain volatile substances. If too large a supply of ink is maintained at the coating nip, undesirable effects can result from evaporation of the volatile material and/or separation of particles from the ink's liquid constituents. Thus, it is desirable to maintain no more than an adequate amount of ink at the coating nip, and it is desirable that this minimal amount of ink be available in a uniform amount along the length of the coating nip.
Exemplary means of supplying ink to a coating interface are shown, for example, in U.S. Pat. Nos. 4,038,442 and 4,142,010. In the former patent, coating ink is directly applied to a substrate by the use of a die-like coating head which includes a number of discharge conduits or ports. In the latter patent, a die-like coating head is also provided. In this case, a runner conduit (48) holds a quantity of the ink, and supplies the ink to a coating slot (38) by way of a comb-like shim (20). This shim operates to control the flow of ink between the reservoir and the coating slot.
U.S. Pat. No. 4,299,186 is similar in its teaching to U.S. Pat. No. 4,142,010.
Copending, and commonly assigned U.S. Patent application Ser. No. 316,367, filed Oct. 29, 1981, now U.S. Pat. 4,387,124 describes a flow manifold which supplies a non-Newtonian liquid to a coating pool by way of a multi-conduit manifold. This manifold is constructed and arranged such that all ink entering the coating pool has experienced the same identical flow history, and is thus of the same viscosity.